The present invention relates to a process for the partial thermochemical vacuum treatment of metallic workpieces.
The thermochemical treatment of workpieces of metals in a gaseous atmosphere of decomposable carbon compounds and/or nitrogen compounds, optionally mixed with other gases, for example inert gases and/or hydrogen is known. For example, DE 41 15 135 C1 describes a process for the treatment of, inter alia, hollow bodies such as injection nozzles or structural parts with bores that are similarly difficult to access. In this process the workpieces are loaded as loose items without any particular arrangement or alignment in the batch receiver. As a result the bore treatment depth is difficult to control, the external surfaces of the workpieces being preferentially treated. If the external surfaces are to be subsequently machined, this becomes difficult or impossible, since after the machining a hardening is out of the question on account of the hardening distortion.
EP 0 818 555 A1 too is concerned with the carburization of hollow bodies with blind holes, though here too the carburization preferentially takes place on the external surface of the hollow bodies.
EP 0 695 813 A2 discloses the use of a plasma with a pulsed voltage of between 200 and 2000 volts for the carburization. Here too however the total external surface of the workpieces is always carburized.
The Applicants"" in-house publication xe2x80x9cVakuumgestxc3xctzte Kohlungsverfahren mit Hochdruck-Gasabschreckungxe2x80x9d, [xe2x80x9cVacuum-Assisted Carburization Processes with High Pressure Gas Quenchingxe2x80x9d] W2004d/9.97/2000/St., discloses complete process sequences both in a single-chamber vacuum furnace as well as in a multi-chamber through flow unit. The treatment of the external surfaces of drive/transmission parts such as gearwheels and shafts is described in particular. EP 0 313 888 B2 specifically relates to high pressure gas quenching for the hardening of steel workpieces.
It is also known to carburize workpieces partially in conventional gas carburization by xe2x80x9csealingxe2x80x9d with a covering paste those external surface regions that are not to be hardened. Such covering pastes are however not suitable either for vacuum processes or for plasma processes since the covering pastes are unable to withstand the ion bombardment of the plasma. Attempts have also already been made mechanically to cover screw threads by encapsulation or plugging, but in this case also xe2x80x9csubcreepagexe2x80x9d of the coverings readily occurs due to the different expansions involved, which often can be eliminated only with difficulty and resulting in damage. Also, the threads that are thermally co-treated are no longer dimensionally accurate after the treatment.
It is known from DE 29 20 719 A1 to carburize in a zone-like manner individual annular workpieces such as gearwheels, coupling parts, running rings for roller bearings and the like so that the zones that are not to be carburized are screened by means of reusable sheathings against the carburization gas. This is achieved for example by covering the front faces of the workpieces with disc-shaped moulded parts of metal or briquetted metal powder that engage via annular flanges partially in the bores of the workpieces, and that are stepped or have an annular groove in order to protect the ends of the workpieces. In each case the largest proportions of the internal and also of the external surface regions are exposed to the carburization gas. Due to the carburization and hardening of the external surfaces a subsequent mechanical treatment, e.g. thread cutting, is made difficult. Although a continuous fabrication involving placing the workpieces on a porous conveyor belt and transportation through a throughput furnace are in fact disclosed, nevertheless this always involves the treatment of individual parts. Injection parts for engines and the non-carburization of the external surfaces of these parts are not disclosed.
It is known from WO 00/58531 A1 when coating workpieces with aluminium and/or chromium and compounds thereof to protect partial regions of the workpieces, for example the seat or roots of turbine blades, against the influence of the coating material by providing these partial regions with reusable masks or caps comprising ceramic components that do not react with the workpieces. However, the xe2x80x9cmaskingxe2x80x9d of individual workpieces and the coating of external surfaces of the said workpieces are always involved. Injection parts for engines are not disclosed, and in particular the non-carburization of all external surfaces of these parts is not mentioned.
Also, it is known from WO 99/13125 to protect a partial length, i.e. the end of tubular workpieces, for example drill elements, against a thermochemical surface treatment by providing the end of the workpiece with a cap that screens the aforesaid partial length against the influence of the thermochemical surface treatment. The largest part of the external surfaces is however subjected to the thermochemical treatment. Here too it is the masking of the ends of individual workpieces that is described however. Injection parts for engines are not disclosed.
From DE 35 02 144 A1 it is known to protect the internal surfaces of annular workpieces comprising plane front surfaces such as slitted piston rings against a nitriding treatment by protecting these internal surfaces with for example a coating of copper, nickel, chromium or tin. By axially arranging in rows and congruently tensioning the front faces of several workpieces against one another on a carrier it can also be achieved that only the cylindrical external surfaces are subjected to the nitriding treatment. This is the exact opposite of the invention, in which all external surfaces are to be protected against a thermochemical treatment. The process is neither intended nor suitable for workpieces other than annular workpieces that can be mounted on one another in a plane-parallel manner.
From DE 28 51 983 B2 it is known in the carburization of hollow bodies with different wall thicknesses, such as for example in the case of nozzles for diesel engines, to encase the surface regions of the thin-walled sections in jackets, in which a carburization process takes place at a lesser intensity than on the remaining surface regions, in order to avoid a so-called xe2x80x9cthrough-carburizationxe2x80x9d and an embrittlement. This also applies to the embodiment in which several thin-walled sections of the nozzles are introduced through bores into a common, box-shaped cavity. For all embodiments it is true however that all surface regions, i.e. also the external surfaces of the workpieces, are to be carburized, and that the surroundings of both the thick-walled and also the thin-walled sections of the hollow bodies participate in a mutually throttled manner, for example via the nozzle bores themselves, in the periodic gaseous exchange in a vacuum furnace. The carburization and subsequent hardening of the external surfaces is extremely disadvantageous for a subsequent metal-cutting machining of the workpieces.
None of the aforementioned publications deals with the following problem:
1. It is difficult if not impossible to cover irregular and/or rough surfaces that have been formed for example by casting or forging processes against the penetration of for example carburization gases.
2. On heating to the conventional temperatures for gaseous treatments, which are carried out at 900xc2x0 C. and above, the covering effect may be reduced or even destroyed by heat distortion, different expansions, etc.
3. Thin-walled extensions of otherwise thick-walled workpieces tend to undergo considerably more severe embrittlement.
4. With the partial covering of workpieces, the boundary between treated and untreated surface regions may be displaced during the treatment as a result of different thermal expansions.
5. Workpieces of relatively large batches, in particular in a mass production run, are exposed to identical process parameters in all predetermined surface regions.
The object of the invention is accordingly to provide a process of the generic type described in the introduction, by means of which several workpieces or batches comprising many workpieces are subjected partially, i.e. only on precisely predetermined internal surface regions, in particular in specific cavities of workpieces, to accurately predetermined process parameters that are at least largely identical from workpiece to workpiece and are reproducible over many treatment cycles. The important point therefore is not only partially to treat the workpieces of a particular batch in a uniform manner, but also the workpiece or workpieces of subsequent batches.
The aforementioned object is solved with the process and apparatus according to the present invention.
The aforementioned object is fully solved by means of the invention, i.e. a process and an apparatus of the generic type described in the introduction are comprehensively improved in that, by means of the process, several workpieces or batches comprising many workpieces can be partially thermochemically treated, i.e. only on accurately defined surface regions in cavities of workpieces. This treatment is effected with precisely predetermined process parameters that are at least largely identical from workpiece to workpiece and that are reproducible over many treatment cycles. The important point in particular is not only to treat the workpieces of a specific batch uniformly and partially in a defined manner, but also the workpieces of subsequent batches, for example in continuous or quasi-continuous processes.
The essence of the invention is accordingly not to treat thermochemically, for example carburize, the external surface of the workpieces, but instead to ensure, wholly or partially, the thermochemical treatment of the internal surfaces.
The invention consists as it were in a reversal of the conventional procedure: it is no longer the largest part of the workpiece surface that is subjected to the thermochemical gaseous treatment, in which relatively small partial regions of the surface(s) are screened and/or insulated against the gaseous treatment, but instead the whole external workpiece surface, except for the internal regions to be treated, are protected against the action of gas by means of the mould body according to the invention. In this connection it is also not absolutely necessary for the mould body according to the invention to surround the workpieces in a gap-free and joint-free manner in a complementary shaping process, but instead it is sufficient to seal, for example in the treatment of the internal space of hollow workpieces, the aforementioned mould body against the ends of the workpieces, optionally with the interpositioning of sleeves, and to leave free several mould cavities between the sealing points in the interior of the mould body that enclose the workpieces and in which no gaseous treatment can take place.
In this way it is possible to treat thermochemically at accurately defined points workpieces of virtually any geometry and/or with irregular and/or rough surfaces that have been formed for example by casting or forging processes, and when heating to the conventional temperatures for gaseous treatments, which are carried out at temperatures of 800xc2x0 C. and above, to reduce or wholly exclude the influences of a thermal distortion, different expansions, etc., on the covering effect. Thin-walled extensions of otherwise thick-walled workpieces are cooled in a more uniform manner and thereby achieve a more favourable internal stress state. The boundaries between treated and untreated surface regions are no longer displaced by different thermal expansions during the treatment. In particular workpieces of relatively large batches are also exposed to identical process parameters in all predetermined surface regions.
The use according to the invention of the mould bodies enclosing the workpieces and the mould bodies per se according to the invention, which form housings as it were and may also be identified as containers, boxes or the like, enables the latter after they have been loaded with the workpieces not only to be transported into and through a treatment plant involving several process stages, but also surprisingly enables the very large range of treatments occurring in practice to be performed, such as heating, carburization (or nitridation), diffusion, quenching and post-treatment in other units (e.g. deep cooling and annealing) without the individual workpieces having to be xe2x80x9cunpackedxe2x80x9d and reloaded. This surprising effect applies in particular to the case of high-pressure gas quenching that is carried out in just a few seconds, as is described for example in EP 0 313 888 B2 and in the company literature of the same Applicants mentioned in the introduction.
Thermochemical gaseous treatment may comprise not only a reduced pressure gaseous treatment without plasma excitation at pressures of up to 30,000 Pa, in which the mould bodies and optionally also the interconnected sleeve members may consist of an electrically non-conducting material such as a ceramic material. Rather, it is in particular also possible to employ plasma treatment processes in which the mould bodies may in this case preferably consist of an electrically conducting material, preferably graphite, so that the mould bodies serve as electrode (cathode) for the plasma excitation. Further details and advantages may be found in the detailed description.
The mold bodies are made of carbon fiber composite (CFC).
Within the scope of further developments of the process according to the invention it is particularly advantageous to employ the following features, either individually or in combination:
in each case at least one surface region of the cavity of the workpiece is screened by means of an inserted sleeve against the thermochemical treatment, whereas at least one further surface region of the cavity is subjected to the thermochemical treatment,
the thermochemical treatment is carried out under the action of a plasma and the mould body consists of an electrically conducting material,
a mould body is used having a plurality of mould cavities for accommodating in each case one workpiece,
the mould body is formed as a housing with an upper part and at least the upper part has openings that communicate with the cavities in the workpieces and through which the carbon-containing atmosphere enters the workpieces,
sleeves are employed between the surface regions of the workpieces not to be treated and the mould body for the purposes of sealing,
a plurality of mould bodies are combined to form a batch,
the process is carried out in the vacuum range between 10 Pa and 3000 Pa, preferably between 50 Pa and 1000 Pa,
the process is carried out with plasma voltages between 200 and 2000 volts, preferably between 300 and 1000 volts,
the plasma is employed as pulses, in which preferably the connection times are selected between 10 and 200 xcexcs and the pause times between 10 and 500 xcexcs,
at least one hydrocarbon from the group comprising methane, ethane, propane and acetylene is selected as carbon-containing gas,
at least one gas from the group comprising argon, nitrogen and hydrogen is added to the carbon-containing gas, the proportion of the at least one hydrocarbon being chosen between 10 and 90% by volume,
graphite or CFC is used as material for the mould bodies, especially if a material is used for the mould bodies that does not exhibit any deformation phenomena at least up to a temperature of 1050xc2x0 C., preferably up to 1200xc2x0 C.,
the plasma-side ends of the at least one mould cavity of the mould bodies opposite the respective workpiece are formed in a plasma-tight manner, and/or
the workpieces within the mould body are subjected
a) before the carburization to a heating process,
b) after the carburization to a diffusion process,
c) after the diffusion process to a high pressure gas quenching,
d) after the high pressure gas quenching to a further treatment involving deep cooling and annealing.
Within the scope of further modifications of the apparatus according to the invention it is particularly advantageous to employ the following features, either individually or in combination:
the mould body is formed as a housing and consists of an electrically conducting material and the workpieces can be enclosed in the mould cavity in such a way that when employing plasma no plasma is formed between the mould body and the workpieces,
the mould body for the treatment of workpieces with cavities that are subjected to a thermochemical vacuum treatment has several openings that communicate with the cavities of the in each case associated workpieces,
the mould body is formed as a housing with an upper part and at least the upper part has several openings that communicate with the cavities in the in each case associated workpieces,
the mould body has a lower part that has several openings and the axes of the openings in the upper part and in the lower part coincide,
a separating groove running along the circumference and that permits a telescopic movement between the lower part and upper part is arranged between the said lower part and upper part,
the plasma-side ends of the openings in the mould body opposite the respective workpiece are formed in a plasma-tight manner,
sleeves are provided that can be inserted between the workpiece and the lower part on the one hand and the workpiece and the upper part on the other hand, and which match the workpiece in such a way that surface regions of the workpieces not being treated are excluded from the thermochemical treatment,
a plurality of mould bodies are combined by a transporting frame to form a batch, in particular the transporting frame has crosspieces for installing mould bodies in a spaced-apart manner adjacent to one another and on top of one another,
graphite or CFC is used as material for the mould bodies, and in particular a material is used for the mould bodies that does not exhibit any deformation phenomena at least up to a temperature of 1050xc2x0 C., preferably up to 1200xc2x0 C., and/or
the mould body is arranged within an evacuable chamber with an inlet for at least one hydrocarbon and is connected as a cathode for the formation of a plasma.
An embodiment of the subject matter of the invention and its mode of action are described in more detail hereinafter with the aid of FIGS. 1 to 6 in conjunction with a thermochemical plasma treatment.